This invention relates generally to following tracks of rotating storage media, such as optical discs, for the purpose of reading or writing data and, more specifically, to servo control loops of which a read head and/or components of a read head are a part, such as those read heads including a light source and photodetector, to follow signal tracks on the media. Removable optical disc applications include playing or recording compact discs (CDs) and digital versatile discs (DVDs) of various specific types.
In optical disc applications, a coherent beam of light is focused to a very small spot on the spinning disc and is caused to follow a spiral optical track. A disc player reads optically encoded data from the track or tracks. Light reflected from the disc, which has been modulated by the optical encoding, is directed onto a photodetector array. Electronic signals from the photodetector array are then processed to retrieve the data stored on the disc track. If in a recorder, the disc track does not yet contain data but is followed by the spot of laser light during recording. The laser light is modulated during recording by the data being stored, and in turn the modulated light is optically recorded on the track in the form of permanent or erasable marks.
An optical system including a laser light source and photodetector array is typically caused to follow the track by a mechanism and motive source that are part of at least one servo control loop. That is, a motive source causes the optics to follow the track and the output of the photodetector array includes a signal from which an error in position of the focused spot of light with respect to the track may be determined. When there is an error, the optics are then moved by operation of the servo control loop in a manner to eliminate the error. There are typically two such control loops used to maintain the focused spot of light on the track of a spinning disc, one that maintains its axial position with respect to the layer of the disc containing the track and the other the spot's radial position on the track.
There is nearly always some degree of radial eccentricity of the track that the radial tracking servo control loop must be able to follow, usually caused by an imprecision in the formation of the tracks on the disc or a lack of alignment of the center of the tracks with the center of the hole that fits onto the spindle of the machine. This repeatable runout can be a considerable source of tracking error. The amount of eccentricity is particularly high for discs that are not manufactured with precision but rather are mass produced at a very low cost. The optical head of a disc machine needs to follow these eccentricities if the data are to be accurately read or recorded. Imprecision of the machine's spindle mechanism may also be a cause of radial eccentricities of the track, which can change over time as bearings and the like wear from use.
A second imprecision that usually exists in following the track is axial eccentricity, where the track moves back and forth along the optical axis of the read head as the disc rotates. This most commonly is caused by the disc not being positioned flat on the spindle, so that it has some tilt with respect to the desired plane of rotation of the disc. It can also be caused by the disc itself being warped. A focus mechanism of the optical head, part of the typical second servo control loop, operates to maintain the spot focused on the track as the disc rotates.
Each of these disc eccentricity and tilt effects result in a periodic variation of track position that has a fundamental frequency equal to the speed of rotation of the disc. In addition to the fundamental frequency (first harmonic), there are usually components of motion at one or more higher harmonics. It is desirable that the tracking and focus servo control loops be able to follow the higher harmonics, as well as the fundamental frequency, so that the optics accurately follow the track while at the same time maintaining stability of the control loops. This can be accomplished by giving the control loop a high gain and a wide bandwidth that accepts all the harmonics but this results in a high degree of noise in the loop. An instability of the loop usually follows from this, however, which is not acceptable.